Encoders are sensors for measuring absolute positions, or relative positions of a component in a system relative to a predetermined reference point. Encoders used to determine absolute position are known as absolute encoders. There are two major types of encoders, i.e. the magnetic encoders and optical encoders. Magnetic encoders work by sensing magnetic field, whereas optical encoders work by sensing changes of light. Generally, an optical encoder comprises a light source, a coding member, and a photo-detector allay. The coding member may be a code wheel configured to rotate about an axis at the center of the code wheel. Thus, encoders with code wheels are also known as rotary encoders. The coding member for linear encoders may be a linear code strip that is movable in a back and forth manner. Accordingly, encoders with a linear code strip are known as linear encoders.
Optical encoders may be divided further into transmissive optical encoders and reflective optical encoders. In the case of transmissive optical encoders, the light source is adapted to generate light that is then illuminated through light windows of the coding member towards the photo-detector array. In the case of a reflective optical encoder, the light source is adapted to generate light that is then reflected through the coding member onto the photo-detector array.
Encoders are widely used in in the field of industrial automation, such as robotics, automatic machines, or other machineries. However, encoders are also commonly used in consumer products, such as printers. In the case of printers, encoders may be used to measure movement of rollers or print heads of printers. Encoders may also be attached to rollers of electronic massage chairs. For industrial use, encoders may offer sensing and measuring capability, enabling closed-loop feedback in motor control systems or other actuators in robotics, automatic machines, or other machineries. Typically, encoders are used to measure distances of a few micrometers or less.
Resolutions of encoders are defined by minimum distance detectable by the encoder. Generally, resolutions of encoders may be determined by the pattern of the coding member and the size of the detectors. For example, the resolutions of optical encoders may be determined by the code strip patterns and the photo-detectors used to detect the light falling on the photo-detector array. One method for providing increased resolution is to utilize an interpolation scheme. Interpolation may be done by multiplying the output frequency. For example, an encoder that produces a full cycle of sine curve if interpolated eight times, may be able to produce eight full cycles of sine curve after 8× interpolation, traveling the same distance.
However, there may be challenges in interpolating signals generated in encoders, because the signals may be distorted due to wobbling of the coding members, misalignment of the coding member and the photo-detector, refraction and other optical properties of light that make the signal a non-ideal sine curve. In addition, the frequency of the signals generated by a single encoder may range from a low frequency, close to a stationary state, to a high speed of hundreds or even thousands of kilohertz. This makes designing an interpolator challenging and complicated, because it may not be easy to design a circuit that functions in precisely the same manner in a low frequency and a high frequency.
Interpolators may be integrated into optical encoders. Alternatively, interpolators may be available as separate, stand-alone devices. For example, in order to improve a motor system with existing working encoders, a designer may elect to utilize an interpolator to work with the existing encoders so that a higher resolution may be obtained.